If you are using the wrong kind of flour (such as bleached as opposed to unbleached) you might be getting a crumb that is more like a cake than a bread. Also, if you slice the bread when it is too hot from the oven, that can also cause it to come apart. Sometimes whole wheat breads also get very crumbly, so you have to find a recipe that can correct for this. Search this blog and see if any of the bread recipes catch your fancy. Try one or two, and see if your problems are solved.